Accessing environment variable inside Dockerfile - App Platform

Hello,

I am running a website on DigitalOcean App Platform. I have couple of environment variables already configured and working fine. But What I am now trying to do is, I want to access a environment variable inside Dockerfile during build time. My use case is like below:

Dockerfile:

CP ./config/staging/conf.d /etc/nginx/conf.d

As you can see, I am trying to make the staging string into an environment variable so that I can use it on both my staging & production branches.

I tried something like this:

CP ./config/${ENVIRONMENT}/conf.d /etc/nginx/conf.d

But it didn’t work. Can someone help me?